The present invention relates to an image sensor, and more particularly to a bolometer infrared image sensor with an improved compensation circuit for compensating fixed pattern noise included in image signals.
Infrared image sensors have dispersions in sensitivity, so-called fixed pattern noise, due to variations of properties of individual pixels. In order to improve picture quality, it is necessary to remove fixed pattern noises included in image signals. Variations of properties of individual pixels are classified into variations in level of biases for every element and variations in sensitivity for every element.
The variations in level of biases for every element have been removed as follows. Under uniform infrared input conditions, fixed pattern noises native to each sensor are first obtained for once storing the same into a memory as fixed pattern noise data so that when any input image data are received, the fixed pattern noise data are fetched from the memory so as to subtract the fixed pattern noise data from the input image data whereby the fixed pattern noises are removed and image data free of fixed pattern noise can be obtained.
On the other hand, the variations in sensitivity for every element have been removed as follows. Output data for quantities of two different incident lights which are uniform in plane are fetched to obtain a difference between them so that the from the difference, gain information is obtained for every element whereby multiplication and subtraction are made for compensation of the variations in sensitivity. This technique is disclosed in the Japanese laid-open patent publication No. 2-107074.
The above conventional compensation circuit needs two circuit configurations for compensation of both the variations in level of the biases for every element and variations in sensitivity for every element. Also, as described above, it is required to execute complicated processes such that output data for quantities of two different incident lights which are uniform in plane are fetched. The compensation of both the variations in level of the biases for every element and variations in sensitivity for every element are carried out in the different two processes.
In the above circumstances, it had been required to develop a novel infrared image sensor capable of compensation of both the variations in level of the biases for every element and variations in sensitivity for every element only by obtaining data one time under uniform infrared ray input conditions.